1. Field of the Invention:
The present invention relates to a process for the concentration of a diluted solution, and in particular an aqueous solution.
2. Description of the Prior Art:
Such a problem is frequently encountered, for example, in the chemical, pharmaceutical and agricultural food industries.
The standard techniques most frequently used involve evaporators which are, in most cases, applied in multiple-effect arrangements so as to limit energy consumption as much as possible; but even under such conditions, the energy consumption is always high, and certainly higher than the theoretical value of the work of unmixing.
Processes and apparatus are known for which the following French patent applications have been filed: FR Pat. No. 75 114 38 (published under No. 23 07 227), FR Pat. No. 76 14 965 (published under No. 23 52 247) and FR Pat. No. 77 07 041. The machines described in these patents can be termed "polytropic machines."
They are constituted by a series of pressure/temperature cells in series in which a working fluid that is present in each cell circulates in the form of saturated vapor in contact with its liquid phase. In addition there are, in at least some of the cells, one or more heating or cooling elements which connect the cells with heat-transmitting fluids which add heat from a generating source or remove the heat required for an area of consumption. Finally, each cell is connected with the neighboring ones in the vapor passage by means of a compressor or a turbine, depending on whether the primary heat entering the process is, on the average, available at a high or a low temperature level, the vapor rising or descending through the pressure/temperature levels. Alternatively, the connection is in the liquid passage which circulates in an opposite direction to the vapor and in equal quantity, through a calibrated orifice, to descend through the pressure/temperature levels or in order to rise through the levels, by a pump. The disclosures of the publications illustrate the structure of such machines.
In the case where the heat-transmitting fluid supplies heat (it is then passing through the series of stages in the direction of decreasing temperatures), a vapor of the working fluid is produced by ebullition of the liquid present in the cell and, in the opposite case some vapor of the working liquid is condensed. Also, the flow rates of vapor and of liquid evolve from stage to stage in accordance with the quantities of heat that are added or subtracted as a function of the Q(T) law in accordance with which the addition or subtraction of heat is effected, i.e. as a function of the dimensioning of the exchange elements.
It must be noted that, in principle, at the interface between two successive cells, the sum of the flow rates of the working fluid entering in the form of vapor or in the form of liquid is always equal to the sum of the flow rates of the same fluid leaving in the form of vapor or in the form of liquid, it always being true that the flow rates of the working fluid as vapor and liquid in opposite directions are equal.
It can be observed that the polytropic machines described in the cited patents can be assembled on the basis of four elementary series, two of which will be used in the following description. The series are:
a compressor series, used for a "condensation process with absorbed work" PA0 a heated compressor series, used for an "ebullition process with absorbed work" PA0 a cooled turbine series, used for a "condensation process with furnished work" PA0 a heated turbine series, used for an "ebullition process with furnished work."
These four elementary types of series each comprise an open endstage through which enter or leave the liquid and vapor flows of the working fluid, and a closed endstage where the working fluid is either completely vaporized or completely condensed.
The following table indicates at the side where the open endstage is located, the inputs and outputs of working fluid in relation to each particular series, as well as the direction of flow of the heat-transmitting fluid.
______________________________________ Direction of heat- transmitting Type of series Open stage Working fluid fluid ______________________________________ Heated compressor higher input: liquid decreasing series temperature output: vapor temperatures Cooled compressor lower input: vapor increasing series temperature output: liquid temperatures Heated turbine lower input: liquid decreasing series temperature output: vapor temperatures Cooled turbine higher input: vapor increasing series temperature output: liquid temperatures ______________________________________
In these systems, the heat-transmitting fluid may pass through several successive stages, or through only a single one. At the limit it is possible to have a heat-transmitting fluid of a different nature for each stage of a given series.
It is also known that the operation of a polytropic machine can be generalized to the case where the flow rates at the entry to the open stage for the liquid and vapor working fluid are different; in such a case, the difference between the flow rates circulating in the two directions is maintained at the value it has upon entry, until it reaches the other endstage, which is thus crossed by a flow of working fluid and is therefore no longer a closed stage; it is said in this case that there is an "open process at both ends," or more simply that there is an open process. We will now describe two examples of the process which will be used within the framework of the invention.
The first process, or process A, is a condensation process with absorbed work, where a vapor flow M+m' enters into the low-temperature open stage; a liquid flow M exits from the same open stage, and a vapor flow m' exits from the high-temperature endstage.
The second process, or process B, is an ebullition process with work furnished to the outside. Into the low-level open stage enters a liquid flow M, and a vapor flow M+m' exits; the vapor flow m' enters into the high-level endstage.
Finally, it is known that in the case where the heat-carrying fluid is identical with the working fluid, the heat exchanges between the two fluids need not be effected through a partition, and that the heat can be exchanged without inconvenience through mixing at each stage; the polytropic process is then called an "open process" without exchange surface; thus, for example, a heated compressor series where the liquid and vapor flows entering the high-temperature open stage have the value M, and where the heat-transmitting fluid entering the open stage and exiting at low-level from the closed stage has the value m, has in principle an operation that is identical to that of a series where the liquid flow entering the open stage is M+m, the vapor flow exiting from the same stage is M, and where a flow m exits at low level.